There has existed an image forming apparatus for outputting an image signal as a visible image to a recording medium such as paper, which adopts a method generally called electrophotography. This apparatus forms an electrostatic latent pattern by optical writing means on a photoreceptor having photoconductivity, and the electrostatic latent pattern is developed by allowing toner which is developing particles to adhere. Thereafter the apparatus transfers the developed image to a recording medium such as sheets so as to form an image signal as a visible image on the recording medium. Concretely, the image signal is converted into an optical signal by a light generator such as a laser, a LED (Light Emitting Diode), and the light is emitted to a photoreceptor which has been charged uniformly so that an electrostatic latent pattern is formed according to light intensity on the surface of the photoreceptor. Next, charged toner is allowed to contact or fly to the electrostatic latent pattern so as to develop it, and the toner image is formed on the surface of the photoreceptor. Successively, after the toner image is transferred to a recording medium such as a sheet by electrostatic attractive force, pressure, or both of them, the toner image is fixed on the recording medium by pressure, heat or both of them.
In addition, as another image forming apparatus, there exists an apparatus which forms a charge pattern according to an image signal on a dielectric drum and obtains a visible image on a recording medium by developing the charge pattern. The above image forming apparatus includes a charged particle flow generator, a charged particle flow control grid and a dielectric drum as a developing substance. Charged particle flow generated from the charged particle flow generator is controlled by controlling a voltage which is applied to the charged particle flow control grid according to an image signal. Then, a charge pattern is formed on the dielectric drum by the charged particles according to the image signal, and the charge pattern is developed by toner so that the toner image is formed on the dielectric drum. Thereafter, the toner image is transferred and fixed in the same manner as in the above-mentioned image forming apparatus.
However, in such an image forming apparatus, after an image signal as an electrostatic latent image is temporarily formed on a developing substance such as a photoreceptor or a dielectric, a toner image is obtained by developing the electrostatic latent image by using toner. Therefore, in the image forming apparatus, a developing substance having a special structure for forming an electrostatic latent image as well as writing means for the electrostatic latent image and charge eliminating means for erasing residual electric charge on the developing substance are required. Furthermore, a constitution that the toner image formed on the photoreceptor is transferred to a recording medium is complex. For this reason, the apparatus is complex and there arises a problem that miniaturization of the apparatus is limited.
Meanwhile, in an image forming apparatus adopting an electrophotographic method, a photoreceptor for forming an electrostatic latent image is required, so there causes a problem of enlargement of the apparatus. Therefore, PCT Unexamined Patent Publication No. 1-503221/1989 (Tokuhyohei 1-503221) discloses a method and an apparatus which open and shut a passage (hereinafter, referred to as gate) where pigment particles pass by means of an electric field in an electrode matrix so as to form an image by the pigment particles on a recording medium. As shown in FIG. 92, in this image forming apparatus, a pair of electrode layers 304 and 305, which are composed of a plurality of wires arranged parallel at uniform intervals, are arranged between a conveyor roller 301 and a sheet 303 so as to form the gate. The electrode layers 304 and 305 are vertically matched with the wires being at right angles to one another. Moreover, a plate electrode 306 is positioned below the sheet 303. The conveyor roller 301 magnetically attracts magnetic pigment particles 302 on its circumference, and an electrostatic field is selectively formed between the conveyor roller 301 and the plate electrode 306 by changing an applied voltage of the wires which constitute the electrode layers 304 and 305. The magnetic pigment particles 302 are carried by the electrostatic field from the conveyor roller 301 through the electrode layers 304 and 305 to the plate electrode 306. The pigment particles 302 are adsorbed on the sheet 303 so that an image is formed thereon.
In addition, as to the arrangement of an electrode section where pigment particles selectively pass in the image forming apparatus, in Japanese Unexamined Patent Publication No. 4-211970/1992 (Tokukaihei 4-211970), as shown in FIG. 93, a gate 413 is surrounded circularly by electrodes 405 so that each electrode 405 which composes the specific gate 413 does not make contact with another electrode 405. With this arrangement of the electrode section, the electrodes 405 do not cross-link, only the gate 413 where pigment particles should pass is opened in connection with matching with image information, thereby making it possible to form an accurate image.
However, in the apparatus disclosed in the above publication, an arrangement that flying of toner from a toner holder to a plate electrode is well controlled and that a high-quality image is obtained has not been sufficiently considered. Therefore, there arises a problem that a good-quality image according to an image signal cannot be obtained.
In addition, as to conventional image forming apparatuses, it is necessary to prepare a keying signal for opening and shutting a gate according to an image signal inputted from an external apparatus, but there does not exist an apparatus with a low-priced and high speed arrangement for converting the image signal to keying data. For this reason, in the case where the conversion process of an image signal is executed by an analog circuit, there arises enlargement of a circuit, and in the case where the conversion process is executed by a digital circuit, burden to the CPU process increases and the process time becomes long.